When a voltage difference between an input voltage VIN and an output voltage Vout is large, a switching regulator, which uses an inductor and a switching transistor to transform magnetic energy stored in the inductor to a DC voltage, is usually employed. However, the switching regulator has a complicated circuit design and a significant noise level.
When the voltage difference between the input voltage VIN and the output voltage Vout is small, a series regulator having a simple circuit design and low noise level is usually employed. The series regulator does not use magnetic energy, and instead directly transforms the input voltage VIN to the output voltage Vout, resulting in a significant thermal loss. In addition, there is a proportional relationship between conductive loss and the voltage difference between the input voltage VIN and the output voltage Vout. To reduce conductive loss, a low dropout (LDO) regulator, which is a type of series regulator that can work when the potential difference is as small as about 0.1 V, is often used.
In recent years, with progress made in forming finer semiconductor integrated circuit devices and lowering the operating voltages thereof, a tolerable power supply voltage range of CPU and other semiconductor devices has become narrower. However, during operation, the load current drawn by such devices may vary significantly corresponding to their operation mode, and so they usually require the variation in the output voltage to be 50 mV or lower even when there is a drastic change in the load current.
In order to meet demand, a broader band low dropout regulator is needed. However, for the conventional low dropout regulator, as the band is made wider, the power consumption rises.